Apparatus for varying the frequency of resonant cavities



March 30, 1948. WIKETQMABIM 2,438,768

APPARATUS FOR VARYING THE FREQUENCY OF REBNTX II T CKV'IT'I'ES" Filed April 28, 1944 to anode voltage 27 [a can tr0l 0r mddlllailhg voltage INVENTOR.

Zyfizb" A2707 5 W W Patented Mar. 30, 1948 w -1 masses APPARATUSLFOR .VARYINGTHE FREQUENCY "OFRESONANT CAVITIES William Afistcwart, Philadelphia;:Pag' -assignor, by 'm'esn'e assignments; itd Philcowfiorporation,

Philadelphia, w-Pah, a corporatione of Y Pennsyl- I -Vania ApplicationApril 28 'B 'OIaims.

The present invention relates to arrangements for varying the frequency ofresonant cavities such asused inultra-highirequency radio cir- 'cuits. The-invention relates further to an apparatus and means for frequency modulatingesell: lators of the type WhOSe' frequency i's fixed' or controlled throughthe agency of an ass'ociated resonant cavity.

Resonant cavitiesare-us'eful in MItra high-frequency techniques for a number of-purpose'si In:

one application of the resonant cavity to nltrahigh frequencysystems the *cavity is" used. in conjunction with an oscillator tube to -supply the essential circuit constants 'a'nd' thereby" to determine the frequency of *osclllation; In this application the cavity-maybe of a generally toroidal shape, and usually surrounds at -least a portion of the-vacuum tube.Suchcavities'are commonly used inconjunc-tion with velocity modulated vacuum tubes"and"generally-= are of, relatively small dimensionsi frequency circuit arrangement it is frequently desirable or necessary to vary the frequenc'y of In such ultra-high the cavity. In'order to tune-the cavity or; in

' other 'Words, to" shift the resonant frequency- 25 thereof, it has been common to insertwarious devices; such as tuning plugs or vanes, -into the cavitythrough suitable openings therein; 'Such "devices'are generally rather-dif icult toconstruct and adjust and hence it has been found-desirable to provide an'alternative' arrangement for-changing the resonant frequency of thecavity without physical adjustment of the cavity structureitself,

I and Without substantial physicahdisplace'ment either of the magneticfieldtherein,'or'alteratioriof the length of thel'electrostatic "lines offorce therein.

In other ultra-high frequency circuit'arrangements resonant cavities are utilized with velocity modulated vacuum tubes, and the frequency: of

the cavity is varied, or modulated, [by providing means for introducing voltage variations into-the supply voltages for the vacuum tube.v -Such arrangements have the disadvantage of-causing;

concomitant power output variations. 5-Therefore, it has'also been foundndesirable to provide an improved methodand apparatus for-frequency modulating the ultraehigh frequency energy associated with cavity oscillating "circuits-Jr.Hereatofore the possible range of such-variation; or in other words the range of" the modulatiomfihas not been as great as desired a'nd hen ceit i's' desirable to providean arrangement whereby the modulation frequency band will cover a-" wid larange' ofigiorrexample-i' several -percent obs-the :center'drequency.

In accordance :Withzthe present: inventionathe -above :mentioned" desirable characteristics and A 5 others whichrsubsequently will become apparent,

' are accomplished 'by'pr'oviding aim-apparatus? by Whi'clnthev frequencyof a cavity. resonatonemay be variedibyielectricalvmeans over: a widezrange O-f-drequencies.

:f It lstthere'forezxan object of: the; present inven- 1 :tion' to? provide? "ansimprovedi arrangementefor I varying. the frequency of a resonant-r cavity.

It is another object oftthe:presentdrrvention tor-provide an:apparatus::for; electrically varying th'e' i'requency. oftza circuit element suchs'as arresonant'i cavity.

It:iszstiilxanother object of the present inven- 1 itiorr torprovide an improved. appanatusion-warylwring-.the trequency of -ultraehigh-frequency radio 0 circuit elements through :a- :relativeiywide range w rof frequencies.

t Still another? i object: ofwthe present invention s'toipro vide are improved arrangement for moduatin'g ultra -high frequency energy over awl ange mounting to several percent of the-central \fre- Stillnai rother object of the' presenta: invention 'sito provide an arirangement-for modulatingsthe I ultras-high frequencyrra-energy of ran;- oscillator ithou'tz appreciably :chang-ingr theepowemoutput fl i'evel.

"i totheriiand further objects of: the present invenition will: subsequently: becomev apparent' by refere11; ence to. the if-ollow-in'gz description?v taken' int'eonnectio-n withthe accompanying drawin'ggr twherein the figure schematically showsan zapparatus for lectrica'lly varying the f-requencyof a ultras'high -"frequency-turied circuit such as a resonanteavity.

orderto illustrate the present invention, it

40* maybe as'smned that it is desired to' contr'il the frequency of a resonant cavity assooiated with a 'velocity niodulated vacuur'ri tubei Inthe drawin'ig there-i's shown aive1ocity -Iriodulated tube I 0 i which may be any, one of a number of the 'gen- 451 erallymsed types of 'ultra -high frequency vacuum tubessuch tubel for example; may comprise "an evacuatedglassenvelope H havinga-reent'r'ant stem on press" F2 through vihichs uitabl'e dead-in ==oonduetors' exte'nd to #the interior or the vac'u'um tube? The vacuum tube may *be provided tvith aheater rearranged to heat-a cathode l4-havlng aplane suriacear'rarigedparallel t'd-th' perforated plane 'surface of 1 a grid 's'tructure I53 The grid structure I5 is spaced from thplan surface of asuitable-'anode l6 asi1fiicient distance to allow for the perforated inner walls I! and I8 which form part of a resonant cavity IS. The perforated wall structures l1 and IS in a tube of the type illustrated are sealed to the inner walls of the glass envelope H. The outer portion of the resonant cavity I9 may be formed of an annular ring-like structure 2! having walls of sufficient thickness to provide adequate mechanical strength, which in turnare connected by any suitable means (not specifically shown) to join with the inner wall structures l1 and I8.

In the past in order to vary the frequency of the resonant cavity i9 it has been common to insert, through suitable openings in the outer wall 2|, plugs or vanes Which either reduce the net volume of the cavity l3, displace the magnetic field therein, or change the length of the electrostatic lines of force. Such devices are frequently 1111'- desirable for mechanical reasons, at least, and are difiicult to adjust accurately. It is desired to provide other means for accomplishing this frequency variation. Furthermore, where it was desired to frequency modulate the energy of the resonant cavity 13, this was accomplished by varying the voltages supplied to the electrodes within the envelope II, which of course had the undesired eflect of changing the power output of the 7 system. In order to vary the frequency of the resonant cavity 19 for either of the purposes just mentioned, or for other purposes, there is pro- -vided,- according to thepresent invention, the apparatus shown to the right of the vacuum tube and the resonant cavity I9.

An ultra-high frequency amplifying apparatus comprising a suitable vacuum tube 23 having an indirectly heated cathode 24, a control grid 25 and an anode 26 is arranged for cooperation with a resonant cavity structure 21. The vacuum tube 23, which in the drawing has been shown as havparticular construction, is but one of a mg a number of ultra-high frequency vacuum tubes which might be utilized as an ultra-high frequency radio amplifying device. The particular vacuum tube 23 illustrated is of the lighthouse type, and has a metal shell base 28 above which is positioned a glass envelope 29 which is secured to the externally extending rim of the perforated grid structure 25. Immediately above the grid ring of the grid 25 there is provided a glass envelope 3! of somewhat smaller dimensions which is secured at its lower rim to the grid ring, and at its upper rim to a washerlike structure secured to the anode 23. The metal shell 28 of the vacuum tube serves as a closure for the open end of an outer cavity member 32. The outer cavity member 32 in effect forms part of the walls of two chambers or resonant cavities. The one cavity 33 comprises thatvolume between the base shell 28 and an intermediate structure 34. The other cavity 35 comprises that volume between the upper portion of the cavity shell 32 and the intermediate structure 34. The intermediate structure 34 comprises an annular member arranged to contact the inner surface of the cylindrical outer shell member32,

and depending from this ring-like portion of the :member 34 is a skirt or cylindrical section of such 7 dimensions as to make contact with the grid flange or ring extending outwardly from the grid struch ture 25. The intermediate member 34 is preferably adjustable axially so that the circuit constants of the apparatus maybe adjusted thereby to determine the frequency of operation. This structure acts as a radio-frequency amplifier. of variable gain.

Suitable conductors interconnect the cathode .2l of tube It).

4, heater with the exterior of the vacuum tube 23 which generally is provided with suitable contact prongs (not shown). The cathode 24 is connected to an adjustable resistor 36, one end of 'which is connected to ground. The grid structure 25 has a D. C. ground return which includes the member 34, the cavity structure 21, the outer conductor 38 of line 40, and the grounded resonant cavity The juncture of the resistor 38 with the conductor connected to the cathode 24 may be coupled through a capacitor 37 to a control or modulating voltage where it is desired to vary the frequency of the resonant cavity l9 at regular intervals or periodically, If

pacitor may, of course, be omitted.

- The lower cavity 33 is coupled to the resonant cavity I9 by means of a coaxial transmission line comprising an outer conductor38 and an inner conductor 39. The end of the coaxial conductor 39 adjacent the resonant cavity I9 is provided with a loop 4| which preferably is arranged to be at a point of maximum magnetic field intensity in the cavity [9, and is so orientated as to have the desired degree of pick-up. Generally this desired degree of pick-up is such that the maximum effect is obtained. At theother end of the coaxial line the inner conductor 39 is capacitively coupled to the cavity 33 by a capacity hat 42 having its surface arranged adjacent the skirt portion of the intermediate member 34. 'At an intermediate point on the coaxial line 40 which comprises the radio frequency filter or choke 46 which is supported by an insulating sleeve 41 within the outer conductor 44. The radio frequency filter 43, which is preferably a quarter-wavelength long, is positioned closely adjacent to the point where the coaxial line 43 makes connection witha suitable aperture adjacent the top of the outer shell structure 32,

The other end of the inner conductor 45 is pro vided with a capacity hat 48 positioned. adjacent to the inner conductor 39 of the coaxial line 40. The junction of the coaxial line section 43 and the coaxial line 40, i. e. the reinjection coupling point, is preferably so positioned that thedistance therefrom to the loop 4|, which distance has been indicated as the dimension A in the drawing, is.such that the reinjection coupling point is at or near a maximum voltage point on the line 40 with respect to the frequency involved. The distance from the reinjection point to the grid 25 of the vacuum tube 23, indicated by the dimensionB, is suchas toprovide a maximum voltage point on the grid, or such as to properly The length of the coaxial line 43, generally indicated by the dimension C, which extends from the reinjection point at the capacity hat 48 to the effective working surface of the anode 26, is such as to provide a maximum voltage point at the reinjection coupling point at or near the operating frequency. The length 0 furthermore is such asto prevent undue regeneration and undesired oscillation.

slow fre-- quency variations are desired the coupling caacco ade Referring new to the modusoperandi' of the system, it may be stated that the invention isbased on-the discovery that a resonant cavity may be tuned by coupling to said cavity a line which is resonant at a'frequency differing somewhat from the natural resonant frequency of the cavity and by terminating said resonant line in'a variableimpedance. The effective resonant frequencyof the cavity then becomes a function of themagnitude of the said terminating impedance. In

the drawing the resonant line consists of that the" natural resonant frequency of the cavity alone. The frequencyto which the line section A is resonant will be referred to hereinafter as the side-frequency.

According to the present invention, the frequency of the oscillations generated by the tube H3 maybe varied by terminating theresonant length of line A in an impedance the magnitude of which may be varied at will to effect the desired frequency changes. ance may, if desired, be any conventional variable resistor suitable'for use at ultra-high frequencies, but'preferably such variableimpedance comprisesan electronically simulated impedance. To this end the structure to the right of the reinjection coupling point comprises means for electronically simulating a desired impedance across the end of the resonant line section A. Employing the dimensions hereinafter recited, such simulated impedance is largely resistive, its magnitude largely dependent upon the gain of the amplifier tube 23; negative resistances may be simulated. It is, however, notoutside the scope of the present invention to employ other line dimensions whose effect will be to simulate reactive impedances, but such is not, at present,- considered to'be the preferred embodiment of the invention.

As was indicated above, the line section A- is preferably an odd quarter of a wavelength long at the side-frequency. The line section B, extending from the reinjection point to the capacity hat 42, is preferably an even quarter-wavelength long at the side-frequency. The line section C, which extends from the reinjection point to the anode supply point 49, has a length, preferably, of an odd quarter-wavelength at the side frequency. The distance D (not shown in the drawing), which may be taken as the electrical distance from the capacity hat 42, through the grid electrode 25, and to the working surface of the anode 25, is preferably an even number of quarterwavelengths, but at least four, at the side-frequency. This electrical length includes the usual phase reversal taking place between the grid and plate of the tube. This amounts, of course, to two quarter-Wavelengths. The distance E, which extends from the anode working surface to the anode supply point 49, is preferably an odd quarter-wavelength The dimensions and E are not individually critical, but preferably the dimension 0 plus the dimension E is an even number of quarter-wavelengths at the side frequency.

If it is desired to vary the frequency of the resonant cavity l9 by manual means, the contact on the adjustable resistor 36 is manipulated. If, on the other hand, it is desired to periodically The variable imped With such a system both positive and controlor to modulate the frequency of the ener y 1 in the cavity 1 913, suitable-source of modulating? voltageis' connected to the coupling capacitor 31 For example, it may be assumed that the coupling capacitor 3'! is connected to a'sine' wave; square:

Wave, or other recurrent p'attermvoltage gen-- erator which thereby will produce a periodically varying gain of the amplifying tube- 23'- which" thereby produces a proportional change in the resonant frequency of the cavity I'9." The total" frequency shift of the resonant cavity 19 will be proportional to the peak voltage applied through capacitor 31 to the amplifying device 21;. The frequency deviation thus produced may be 1618.

tivel'y great, for example of the order of severe percent of the entire frequency.

While for the purpose of illustratingand' .de-.

scribing the present invention certaini specific apparatus has beendisclosed, it is tobe understood that I do not wish to be limited'thereto since obviously. other equivalents mightbe. utilized and various alterations in the circuitarrangement and in the instrumentalities employedmightbemade as may be commensurate with the scopeof, the. invention defined in the appended claims.

I claim:v

1. In an. ultra-high. frequency circuit the combination comprising an ultra-highfrequenoyi oscillating circuit the frequencyofwhich:- is to provide a reinjection oupling point: at a voltage point on said-first coaxialline.

2. In. an ultra-high frequency circuit, the com--. bination comprising an: oscillating circuit the: frequencyof which is to-be'variedian ultra-high frequency, amplifier including a vacuum: tube having a grid-to-cathode cavity and-aagrid etm:

anode cavity, a coaxial line interconnecting. said oscillating. circuit with said grideto-cathode cavity, and a second coaxial-dineinterconnecting said grid-to-anode cavity withan-intermediate point onsaid first coaxial: line. to provide a rein---v jectioncoupling point having a maximum vol-teage at or near the operating-frequency,

3. In an ultra-high frequency circuit, the combination comprising an oscillatin circuit the frequency of which is to be varied, an ultra-high frequency amplifier including a vacuum tube having a grid-to-cathode cavity and a grid-toanode cavity, a coaxial line interconnecting said grid-to-cathode cavity with said oscillating circult, and a second coaxial line interconnecting said grid-to-anode cavity with an intermediate point on said first coaxial line so as to provide a voltage point on the grid of the amplifying vacuum tube.

4. In an ultra-high frequency circuit, the combination comprising a resonant cavity the frequency of which is to be varied, an ultra-high frequency amplifier including a vacuum tube having a plurality of cavities, a coaxial line interconnecting one of said cavities with said first mentioned resonant cavity, and a coaxial line interconnecting the other of said latter cavities with an intermediate point on said first coaxial line, said latter coaxial line being capacitively coupled to said first coaxial line, said latter coaxial line being of such length as to minimize the possibility of regeneration and oscillation.

5. In an ultra-high frequency circuit, the comaxial line interconnecting one of said cavities with said first mentioned cavity, said coaxial line being inductively coupled to said first mentioned cavity and capacitively coupled to said other cavity, and a second coaxial line interconnecting the other of said amplifier cavities with an intermediate point on said first mentioned coaxial line so as to provide a reinjection coupling point at a voltage point on said first coaxial line.

6. The combination-comprising an oscillating circuit having a resonant cavity the frequency of which is to be varied, an ultra-high frequency amplifier including a vacuum tube and a gridto-cathode cavity and a grid-to-anode cavity, a coaxial line connected by a loop to said oscillating circuit resonant cavity, capacitive coupling means connecting said coaxial line with said grid-to-cathode cavity, a second coaxial line connected between .the anode of said vacuum tube and an intermediate point on said first caxial line, said latter coaxial line having an inner conductor capacitively coupled to they inner conductor of said first coaxial line, and means for controlling the amplification of said amplifying vacuum tube'.

7. In an ultra-high frequency circuit, the combination comprising an ultra-high frequency resonant cavity the frequency of which is to be varied, an energy pick-up means positioned within said cavity, an ultra-high frequency amplifier circuit, a coaxial transmission line interconnecting said pick-upmeans with said amplifier circuit, a second'coaxial transmission line interconnecting said amplifier circuit with an intermediate point on said first coaxial line to re-inject amplified ultra-high frequency energy into said first line, and means for varying the gain of said amplifier circuit thereby to vary the operating frequency of said resonant cavity.

8. An ultra-high frequency apparatus adapted to control the operation of another circuit, comprising an amplifier tube having a resonant cavity structure, an adjustable common partition provided within-said cavity structure to form two cavities, one cavity being arranged as a grid-tocathode cavity and the other cavity being ar- 8 ranged as a grid-to-anode cavity, a transmission line capacitively coupled to said grid-to-cathode cavity, said transmission line being adapted to be connected to the circuit to be controlled.

. second transmission line interconnecting said grid-to-anode cavity with a point on said first 'with said modulating voltage.

transmission line so as to re-inject amplified ultra-high frequency energy into 'said first line, and means for controlling the gain of said amplifier thereby to control the operating frequency of said controlled circuit. 7

9. In an ultra-high frequency circuit, the com bination comprising a cavity type oscillating circuit the frequency of which is to be controlled, an ultra-high frequency amplifier including a vacuum tube having a grid-to-cathode cavity and a grid-to-anode cavity, a coaxial line interconnecting said grid-to-cathode cavity with said cavity type oscillating circuit, said coaxial line being inductively coupled thereto and being capacitively coupled to said grid-to-cathode cavity, a second coaxial line interconnecting said. grid to anode, cavity with an intermediate point on said first coaxial line so as to re-inject amplified ultra-high frequencyenergy therein, means for varying the gain of said amplifier thereby to control the operating frequency of said oscillating circuit, and means for applying a modulating voltage to said amplifying tube to vary the frequency of said oscillating circuit in accordance WILLIAM A. STEWART.

REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS Number Name Date 7 2,200,986 Fraenckel May 14, 1940 2,213,103 Gluyas Aug. 27, 1940 2,213,104 Gluyas, Jr. Aug, 27, 1940 2,241,976 Blewett May-13, 1941 2,280,026 Brown Apr. 14, 1942 2,280,824 Hansen Apr. 28, 1942 2,312,919 Litton Mar. 2, 1943 r 2,329,779 Nergaard Sept. 21; 1943 2,349,811 Crosby May '30, 1944 

